Can the output of DC/DC converters connect in series?

Yes. As shown in below figure. It’s a positive and a negative output voltage  connecting through two converter’s output in series. It is recommended that the  output to connect with a reverse diode. The withstand voltage of the diode needs to be higher than the total output voltage of two converters.

power supply - Design considerations when combining multiple DC DC converter with the same input, but different output - Electrical Engineering Stack Exchange

Connecting Power Supplies in Parallel or Series for Increased Output Power

Figure 1 from A modular DC-DC converter with collapsible input voltage of series connected modules without additional bypass switch

Input-Series Output-Series (ISOS) DC-DC converter circuit diagram.

Electronics, Free Full-Text

Series-Connected Boost Regulators - Tech Briefs

a) Parallel dc-dc converters with different output voltage, and (b)

Connecting DC-DC Converters in a Series to Obtain Various Output Voltages

Serial connection of converters: (a) serial inputs and outputs, (b)

2 dc to dc converter in series? - General Electronics - Arduino Forum